1. Field of the Invention
The invention relates to an adsorber element for a heat exchanger, which contains as a central component a heat-conducting solid body, on the surface of which a sorption material for a vaporous adsorbate and on the outer surface of which a fluid-tight foil, or composite film, is arranged.The invention further relates to an adsorption heat pump or adsorption refrigerator that contains at least one adsorber element of this type.
2. Description of Background and Other Information
With many technical adsorption processes, the power with which the adsorption heat can be removed via a heat exchanger plays an important role. This applies analogously to the heat transfer efficiency in heating the adsorbent via a heat exchanger for the desorption/reconditioning of the adsorbent (usually referred to below as sorption material). These power characteristics are of central importance for adsorption heat pumps and adsorption refrigerators and related adsorption applications in energy and climate control technology.
Various companies, e.g., Vaillant, UOP, Mitsubishi and SorTech, are currently working intensively on concepts for adsorption heat pumps and adsorption refrigerators to increase the power density related to the construction volume.
In physical terms, this hereby relates to the problem of optimizing the heat and matter transfer in the heat pump. In a microporous solid (adsorbent/sorption material, e.g., a zeolite or a silica gel) the vapor of the working fluid (adsorbate, e.g., water, methanol or ammonia) is adsorbed, wherein heat is released.
For an improved adsorber element, therefore, a good thermal link of the microporous adsorbent/sorption material (e.g., a zeolite) to a heat-exchanger surface or to a heat transfer fluid flowing on the other side of this boundary layer is necessary.
DE 10 2005 037 708 A1 and DE 10 2005 037 763 A1 describe a compact structure, with which an improved heat transfer is achieved from the heat exchanger plate to the heat transfer fluid. This is achieved either by means of a cylindrical heat exchanger or by means of a plate heat exchanger in which the heat transfer fluid flows through channels in the heat exchanger plate.
DE 101 19 258 A1 describes an adsorber structure in which the adsorbent is placed on the fins of a finned tube in one layer in the form of granules. A better thermal contact of the sorption material to the heat exchanger can be obtained with a structure according to U.S. Pat. No. 6,102,107. The adsorber is hereby designed as a fin coil heat exchanger, that is as a bundle of parallel plates that are pushed through a tube bundle in a perpendicular manner. The heat transfer fluid flows in the tubes and the sorption material is applied to the plates as a layer on both sides. The sorption material is hereby bonded in a polymer foil. With an adsorber design of this type a good thermal link of the sorption material to the heat exchanger surface can be obtained such that the heat transfer to the heat transfer fluid in the heat exchanger becomes the limiting factor for the achievable power density of the heat pump.
Through an adsorber design of this type, however, the power densities are not yet achieved that are required for certain applications (e.g., for car air-conditioning). In order to further increase the power density of adsorption heat pumps and, at the same time, to render possible a high efficiency (COP, performance factor), new concepts are necessary for the design of the adsorber.
The (thermal) COP (Coefficient of Performance) for a refrigerator is understood to be the ratio of useful cold obtained and the driving heat required for this; for a heat pump the COP is defined as the ratio of the useful heat obtained (at the average temperature level) to the driving heat (at high temperature level). For practical application additionally a consideration of the overall energy expenditure is necessary, including the electric consumption of pumps, etc.
A problem in addition to the above-referenced problems is the limitation of the adsorption speed by the transport of the gaseous and/or vaporous adsorbate to the micropores or mesopores of the solid sorption material, where the adsorption heat is released. This problem occurs in particular with adsorbates with low vapor pressure, such as, e.g., water or methanol. In particular with thick layers of the sorption material, a high diffusion barrier results for the adsorbing gas, depending on the structure of the layer.
According to WO 02/45847 one solution approach is to achieve a sufficient vapor-permeability of the sorption material layer by bonding the sorption material micro-particles into a vapor-permeable polymer matrix with good thermal conductivity. DE 101 59 652 C2 describes the insertion of sorption material into a foam-like matrix of metal.
Another approach to a solution—at least for zeolite sorption materials—is described in L. G. Gordeeva et al. “Preparation of Zeolite Layers with Enhanced Mass Transfer Properties for Adsorption Air Conditioning” in Proc. of the Int. Sorption Heat Pump Conf. ISHPC, Shanghai, China, Sep. 24-27, 2002. Here, a suitable pore former is added during the synthesis of the sorption material layer, which is subsequently burnt out or washed out, in order to create additional channels for the vapor transport.